1. Field of the Invention
The present invention relates to an ink jet recording apparatus for recording characters, figures or images on a recording medium by jetting ink droplets, utilizing electrostatic force and air streams.
2. Description of the Prior Art
Hitherto, the ink jet recording apparatus utilizing electrostatic force and air streams is known in USP 4,403,234 (EP 63,853), USP 4,403,228 (EP 61,327), USP 4,736,212 and so on. The conventional ink jet recording apparatus utilizing electrostatic force and air streams is described as follows, referring to FIG. 1.
In FIG. 1, a body 13 is provided with an air nozzle plate 2 formed of an insulating material. The air nozzle plate 2 has a plurality of air discharge channels 1. An ink nozzle plate 14 is provided in parallel with the air nozzle plate 2 and has a plurality of ink discharge channels 4 which are arranged facing the plurality of air discharge channels 1, respectively.
Convex part 17, projecting in the direction of the air discharge channel 1, is formed around the ink discharge channel 4. Between the convex parts 17 neighboring each other, a projection 5 is formed on the ink nozzle plate 14 in order to stabilize air flow. Air flows into an air supply passage 8 from an air source 3, is made uniform in a circular air chamber 9, and further flows into an air passage 7 between the air nozzle plate 2 and ink nozzle plate 14 and finally flows out from the air discharge channels 1.
The air expands at the air discharge channel 1 and therefore a sharp air pressure gradient appears in a space between the ink discharge channel 4 and the air discharge channel 1. An ink chamber 10 neighboring the ink discharge channels 4 is connected to an ink tank 11 through an ink supply passage 6. Ink in the ink tank 11 is applied with pressure by the air pressure of the air source 3, the air pressure being regulated by an air pressure regulating valve 16.
The reason for the air pressure regulation is that it is necessary to maintain a meniscus of ink formed at the ink discharge port 4 static by nearly equalizing the air pressure near the ink discharge channel 4 to the ink pressure of the ink discharge channel 4, or the ink tank 10, when the ink jet recording apparatus is not driven.
Bias electric source 24 is connected to a common electrode 12 provided around the air discharge channels 1 and a plurality of signal sources 22 are connected to control electrodes 15 provided on the surface of the ink nozzle plate 14 facing the ink chamber 10, around the ink discharge channels 4. Since the ink is conductive, the sum of the bias voltage and the signal voltage is applied between the common electrode 12 and ink of the ink discharge channel 4. The meniscus of ink formed at the ink discharge channel 4 is extended toward the air discharge channel 1 by the electrostatic force produced by the voltage.
Further, since there is the sharp pressure gradient in the space between the ink discharge channel 4 and the air discharge channel 1, when the ink meniscus of the ink discharge channel 4 is the proper length, the ink meniscus jets out from the air discharge port 1.
The conventional ink jet recording apparatus has the problem that ink jet volume decreases when atmospheric (room) temperature is 25.degree. C. and relative humidity is 60% or more. Reasons for the ink jet volume decrease are described as follows. FIG. 2(a) shows the electric field when ink jets normally. The air nozzle plate 2 is made of photosensitive glass with a dielectric constant of 6.54.
On the other hand the dielectric constant of air is about 1 and therefore the strength of the electric field in the air is larger than that of plate 2. The electric field which is strong and therefore particularly contributes to the extension of the ink meniscus 21, is shown by electric line of force 18.
The electric line of force 18 is produced through the air discharge channel 1, from the common electrode 12 to ink meniscus 21. Since the direction of the electric line of force 18 and the direction of ink jetting are the same, the electric field effectively extends the ink meniscus 21. FIG. 2(b) shows the electric field when ink jet volume decreases. When the relative humidity is 60% or more, water (moisture, vapor) 20 is adsorbed into the wall of the air discharge port 1.
The specific resistance of the water is 2.5.times.10.sup.7 .OMEGA. cm, namely, conductive, so that the voltage of the water 20 becomes equal to that of the common electrode 12. In FIG. 2(b), the line of electric force 19, which indicates the strong electric field, is produced between the ink meniscus 21 and the water 20 which is nearer to the ink meniscus 21.
In such a case, the direction of ink jet and the direction of the line of electric force are not equal to each other, and therefore the force of the electric field is not concentrated. As a result, the ink meniscus 21 is not efficiently extended and the ink jet volume decreases.
Further, since the electric field is not stable, the ink volume varies. The reason why the water 20 is adsorbed is described as follows. Considering the air discharge channel 1 with air flowing therethrough, the air pressure decreases in the air discharge channel 1 and air volume expands when the air at 0.12kg/cm.sup.2 in the air passage 7 exits to the atmosphere (0 kg/cm.sup.2). When the air rapidly expands, the air absorbs heat from the wall of the air discharge channel 1, and therefore the temperature of the wall of the air discharge channel 1 decreases. Since the temperature of the wall of the air discharge channel 1 is lower than or equal to the flowing air, the wall of the air discharge port 1 tends to adsorb water from the atmosphere. Table 1 shows conditions of temperature and humidity where the ink volume decreases, varies and becomes unstable in a thermo-hygrostat.
TABLE 1 ______________________________________ atmosphere relative humidity temperature in the room ______________________________________ 20.degree. C. 65% or more 25.degree. C. 60% or more 30.degree. C. 55% or more 40.degree. C. 60% or more ______________________________________
The air flowing out from the air discharge channel 1 is air from the room supplied from the air source 3, which is, for example, a diaphragm-type air pump. Therefore, when the relative humidity of the room is 55%-65% or more, the volume of the adsorbed water 20 increases, the ink jet volume decreases and the ink jetting becomes unstable.